This invention relates to liquid metal cooled fast flux nuclear reactor core assemblies and, more particularly, to a nuclear reactor spacer grid formed by the interconnection of grid strips and having a hydraulic spring and to a ductless core component employing this spacer grid.
Cores designed for use in liquid metal cooled fast nuclear reactors typically include closely packed hexagonal fuel assemblies whereby the core approaches the configuration of a right circular cylinder. Typical fuel assemblies include a plurality of fuel rods of circular cross section bundled in a triangular array within a full length thin wall duct. The fuel rods are supported at one end, allowing free axial expansion, and laterally supported along their lengths by wire wraps about the rods and/or by egg-crate type grid structures positioned at selected locations along the assembly length within the duct. Such full-length ducted assemblies limit coolant cross flow among adjacent fuel assemblies, increase the amount of metal in the core, thereby lessening nuclear efficiency, and increase the pressure drop through the core which further decreases efficiency. These adverse characteristics can be avoided by use of open-lattice fuel assemblies which eliminate the full length duct in favor of positioning grid structures at selected axial positions, as well known in the generally square fuel assemblies of light water reactors having fuel rods positioned in a square array.
One form of ductless fuel assembly is described in U.S. Pat. No. 4,285,771 issued to Robert E. Downs on Aug. 25, 1981 and assigned to Westinghouse Electric Corporation. The fuel assembly described therein includes coextending fuel rods of circular cross section arranged in a triangular rod array with the bundle of rods approaching the configuration of a regular hexagon. At each corner, one or more rods is removed and there is located a strong back, generally coextensive with the fuel rods, which is the axial support for the assembly. The outer periphery of the strong back is also the area of contact among adjacent assemblies. While the strong back structures collectively contain less metal than the thin walled duct which they replace, the amount of metal in the strong backs remain significant. Also each strong back occupies a position which could be occupied by one or more rods containing nuclear material.
The egg-crate type grid structures currently employed for laterally supporting rods containing nuclear material within a nuclear core assembly for a liquid metal cooled reactor are formed by the interconnection of grid strip members. However, the method currently used to interconnect the grid strip members to form a spacer grid having cell members therewithin results in approximately half of the walls of each cell member being formed by portions of two grid strip members. The variations in thickness of the grid strip members must be within a certain tolerance. In the walls of the cell members which are made of portions of two grid strip members, the combined variations in grid strip thickness of the two grid strip member portions which make up the walls must be within the same tolerance as the portion of the grid strip members forming single thickness walls. What is needed is a method of manufacturing a grid structure which will overcome this tolerance problem and provide a laterally strong grid structure which does not overly restrict flow of coolant through the core component.
Within each cell member formed by the interconnection of grid strip members in the grid structure, a nuclear material containing rod must be laterally supported. This lateral support generally takes the form of hardstop members disposed on two of the walls of the cell member and a spring member disposed on the third wall forcing the rod against the hardstop members. The force of the spring is derived from bending of the metal. During operation of the nuclear reactor, exposure to radiation and high temperatures can result in spring relaxation. This lessening of the biasing force of the spring can permit vibration of the rod within the cell member, causing damage to the rod. What is needed is a spring for providing lateral support for a rod in a nuclear reactor spacer grid which does not have a lessening bias force after continual exposure to radiation and high temperatures.
It is thus desirable to provide an open lattice core component for a liquid cooled, fast neutron flux reactor so as to achieve, among other advantages, coolant cross flow among core components. It is further desirable to provide a grid structure for use in this core component which provides sufficient lateral constraint for the nuclear material containing rods, does not overly restrict coolant flow through the core component, and which has cell member walls formed from a portion of only one grid strip member to avoid tolerance problems. It is also desirable to provide within each cell member of this grid structure a spring for laterally constraining the nuclear material containing rod which is not subject to spring relaxation by prolonged exposure to radiation and high temperatures.